Polyether polyols are used as materials for rigid polyurethane foams. They are conventionally produced by a method of ring-opening polymerization of cyclic ether compounds such as ethylene oxide, propylene oxide or tetrahydrofuran with active hydrogen compounds as initiators. Because the cyclic ether compounds possess high activity, the reaction proceeds under relatively mild conditions. Further, the polyether polyols obtained by this process have narrow molecular weight distribution. However, the cyclic ether compounds have a low boiling point and high toxicity, and this makes the storage and handling thereof during the synthesis difficult.
With respect to polyether polyols obtained from the cyclic ether compounds, the average number of hydroxyl groups cannot be more than the number of active hydrogen atoms in the initiators because of the nature of the reaction. In some cases, the average number of hydroxyl groups can be reduced by side reactions during the reaction. Polyether polyols for rigid polyurethane foams require a large number of hydroxyl groups, and therefore the production thereof should involve hydroxyl-rich initiators such as pentaerythritol, sorbitol or sugars. However, the hydroxyl-rich initiators are frequently solid, and handling thereof as initiators is difficult.
On the other hand, polyether polyols may also be produced by dehydration condensation of alcohols. According to this method, the number of hydroxyl groups per polyether polyol molecule may be theoretically increased by using polyol compounds having three or more hydroxyl groups in the molecule as the alcohol. With respect to the polyether polyols obtained from dehydration condensation of polyol compounds having three of more hydroxyl groups in the molecule, JP-A-H02-172938 and JP-A-H07-216082 disclose polyglycerins obtained by dehydration condensation of glycerins in the presence of alkalis. When polyglycerins are used as polyol components for rigid polyurethane foams, the polyglycerins should have a high degree of condensation. However, the polyglycerins show very high viscosity when the degree of condensation is high. It is difficult to use such polyglycerins alone as polyol components for rigid polyurethane foams, and the polyglycerins should be mixed with other polyols to a polyglycerin concentration of 30 wt % or less (JP-A-2004-359891).    Patent Document 1: JP-A-H02-172938    Patent Document 2: JP-A-H07-216082    Patent Document 3: JP-A-2004-359891